1. Field of the Invention
The invention relates to elastomeric acrylic resins (EARs) comprised of acrylic acid ester and macromonomer moieties having a glass transition temperature Tg above 60.degree. C.
2. Discussion of the Background
The general usage of the term "elastomer" is to describe materials which can be stretched to at least twice their initial length by application of a moderately low force at room temperature and higher temperatures, and which, after the force is released, return quickely and practically completely to their original size and shape. For a long time the term was synonymous with "rubber" (see Stoekhert, G., "Kunststoff-Lexikon", 6th Ed., pub. Carl Hanser Verlag, Munich).
The prerequisite for elastomeric properties is regarded to be the presence of macromolecules with an extensive flexible chain structure, and in a state distinctly above the glass transition temperature Tg of the macromolecules. Additional prerequisites include a low degree of crystallization of the macromolecules in the non-deformed state, and intermolecular crosslinking of individual chains to form a three-dimensional network which is indissoluble. (See Kirk-Othmer, 1979, "Encyclopedia of Chemical Technology", 3rd Ed., Vol. 8, pub. J. Wiley, pp. 446-469.)
Typical acrylate elastomers such as are currently used in a wide range of industrial applications are polymers and copolymers of acrylic acid esters, with ethyl acrylate and butyl acrylate as the main components. They are particularly resistant to oils and have relatively high thermal stability. The principal disadvantage of such acrylate elastomers is considered to be the fact that they become stiff upon cooling, and are brittle at relatively high temperatures comparatively speaking.
In addition to principally radical-mediated crosslinking of polymers with low glass transition temperatures, elastomers can be produced also by polyaddition reactions; e.g., elastomers based on polyurethane (PUR).
Beside covalently crosslinked PURs, PURs are available which are "physically" crosslinked. Such elastomers are thermoplastically processible.
Whereas with classical "rubber" the crosslinking (i.e., vulcanization) is a slow, irreversible process, which occurs under heating, in the "thermoplastic elastomers" there is a transition from the moldable melt to the elastomeric solid, upon cooling, which transition proceeds quickly and reversibly. (See Mark, H. F., et al., 1986, "Encyclopedia of polymer science and engineering", 2nd Ed, Vol. 5, pub. J. Wiley & Sons, pp. 416-430.)
According to the prevailing view, "thermoplastic elastomers" comprise multiphase systems wherein the phases are intimately interdispersed. In many cases (chemical) bonding via block- or graft copolymerization may play a role, whereas in other cases it seems apparent that a high degree of dispersion is sufficient. As a rule, at least one of the phases is comprised of a polymer material which is "hard" at room temperature but liquid upon heating. A second phase is as a rule comprised of a "soft" material which has rubberlike elasticity at room temperature. The most industrially important materials are thermoplastic block copolymers the "hard" segments of which are comprised of, e.g., polystyrene, polysulfone, polyester, polyurethane, or polycarbonate, and the "soft" segments of which may be comprised of polybutadiene, polyisoprene, polyethylene-butylene copolymers, polydimethylsiloxane, or polyethers. Transparent thermoplastic elastomers based on polyacrylate esters with polystyrene graft branches have been described by G. O. Schulz and R. Milkovich (1982, J. Appl. Polym. Sci., 27:4773-4786). The synthesis of these polyacrylate esters with polystyrene side chains proceeds on the basis of styrene macromonomers such as obtained with very narrow molecular weight distribution according to R. Milkovich et al. (U.S. Pat. No. 3,786,116). In such copolymers the "crosslinking" can be interpreted as association of glass-like or crystalline "hard" blocks into the associated polymer structure.
A factor common to all of the above-mentioned thermoplastic elastomers is that they have very high stress at failure (according to the test of DIN 53 455) but relatively low extensibility. Their weather resistance is also unsatisfactory.
The elastomers according to the known art belong to a variety of polymer classes, and they display the technical weaknesses inherent in these classes.
The "thermoplastic elastomers" are generally inferior to the corresponding vulcanized products in a few quality-related parameters, e.g. compressibility, solvent resistance, and retention of shape under heating. (See Mark, H. F. et al., "Encyclopedia of polymer science and engineering", Vol. 5, loc. cit.)